A pixel in a liquid crystal display (LCD) generally includes a pair of spaced apart pixel electrodes having liquid crystal material disposed therebetween. Thus, each pixel constitutes a capacitor in which electric charge can be stored. The charge stored in a pixel results in a voltage potential across the opposing electrodes and an electric field across the liquid crystal (LC) material. By controlling the amount of charge stored in pixels across an array of such pixels, the properties of the liquid crystal (LC) material can be controlled to obtain a desired light influencing effect or image which is displayed to a viewer.
In LCDs, it is necessary to update the condition of each pixel at a regular interval (i.e. at a given frame rate). This is because pixels can retain or store applied charge potentials for only a finite time. Updating is further required in order to change the image to be displayed to the viewer. The ability to rapidly transfer to, and store electric charge in, pixels and to efficiently retain the stored charge therein for a frame period is thus important.
Metal-insulator-metal (MIM) and other non-linear resistant diode LCDs are easier to manufacture than TFT LCDs and conventional diode LCDs. A typical MIM electronic matrix array requires between two and four thin film layers and photomask steps, as compared to six to nine thin film layers and photomask steps for TFT arrays. Patterning of most MIM arrays can be achieved with less stringent overlay accuracy and resolution requirements, than is required for TFT arrays. As a result, less expensive photoexposure equipment, such as scanning projection aligners, can be used, that have more than twice the throughput and cost less than half as much as flat panel steppers. Despite their low production costs, MIM and other non-linear resistive thin film diode driven LCDs are not widely used. This can be attributed to the inferior performance of typical MIM LCDs with regard to gray shade control, image retention, response time, and maximum size and resolution as compared to TFT LCDs.
Accordingly, there exists a need in the art for an improved backlit thin film diode (e.g. MIM) LCD, which is cheaper to manufacture, less susceptible to image retention and gray scale problems, has high transmission, and has good resolution.
Furthermore, conventional backlit dual MIM VGA LCDs have a pixel aperture ratio of about 50% (e.g. for a 10.4" VGA LCD). There exists a need in the art for a higher pixel aperture ratio in backlit transmissive thin film diode (TFD) LCDs.
Still further, reflectance characteristics under ambient lighting of conventional backlit MIM LCDs is less than desirable, and may not be sufficiently low for automotive applications. Thus, there exists a need in the art for a thin film diode inclusive backlit LCD which has reduced ambient reflectance under ambient lighting conditions.
Yet another problem with conventional silicon nitride MIM LCDs is their rather high amount of photosensitivity due to the MIM devices. High photosensitivity can limit contrast ratio in certain applications. Thus, there exists a need in the art for a thin film diode (TFD) inclusive LCD having reduced photosensitivity.
As shown in prior art FIG. 1, another problem with conventional MIM devices is breakdown voltage of MIM is device at the edge(s) of the bottom electrode. This is currently limited by the step coverage of the semi-insulator layer over the edge of the bottom electrode and the thickness and profile of the bottom electrode and the thickness of the semi-insulator. In this conventional FIG. 1 thin film diode, the SiN.sub.x (silicon nitride) semi-insulator or insulator layer makes a step over the edge of the bottom electrode. This is a weak point in the TFD where destructive breakdown can occur at high voltage, i.e. the device is sensitive to ESD and cannot always operate reliably at desired select voltages. This step coverage can be a problem, and it is desirable for this problem to be solved.
Yet another problem with conventional backlit MIM LCDs is the large number of processing steps needed to make the complete display, including both the active and passive substrates. The MIM devices are on the active substrate, and require a number of processing steps. Meanwhile, the color filters are on the passive or opposite substrate, and also require a number of processing steps to manufacture. The combination of steps required to make both plates (i.e. substrates) is undesirably large. Accordingly, there exists a need in the art for a liquid crystal display including a method of making same having a reduced number of processing steps.
U.S. Pat. No. 5,521,731 discloses a reflective type liquid crystal display (LCD) which is driven by an array of MIM diodes. There is no backlight. For each of the MIM diodes, the lower diode electrode functions as a wiring element. Unfortunately, the display of the '731 patent is undesirable for at least the following reasons: (i) the color filters and MIM diodes are on opposite substrates thereby increasing the number of processing steps necessary to make the display; (ii) the display is a reflective type display (without a backlight) and thus non-transmissive [reflective LCDs generally have poorer image quality than backlit transmissive LCDs]; (iii) no black matrix for reducing reflections in ambient conditions is provided; (iv) certain embodiments utilize cholesteric liquid crystal; (v) only one MIM is provided per pixel which results in inferior image quality; (vi) an extra insulating layer is required between the MIMs and the pixel electrodes, which increases the number of steps needed to make the display; and (vii) the pixel electrodes associated with the MIMs are of an opaque reflective non-transparent material such as aluminum which would be undesirable in a transmissive display.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.